[PATCH] libstdc++: Allow 'configure.host' to modify 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'

[Thomas Schwinge]

Hi Jonathan!

Sorry for not providing more context initially.

On 2025-02-26T10:50:11+0000, Jonathan Wakely <jwakely@redhat.com> wrote:
> On Wed, 26 Feb 2025 at 10:47, Jonathan Wakely <jwakely@redhat.com> wrote:
>> On Wed, 26 Feb 2025 at 10:19, Thomas Schwinge wrote:
>> > In particular, 'GLIBCXX_ENABLE_CXX_FLAGS' shouldn't overwrite 'EXTRA_CXX_FLAGS'
>> > (and prepend any additional '--enable-cxx-flags=[...]').
>>
>> This seems good, but why prepend instead of append here?
>> If there are important flags passed down from top-level configure that
>> shouldn't be replaced by the libstdc++ --enable-cxx-flags option, can
>> we mention that in the new comment in acinclude.m4?
>
> Oh sorry, they're more likely to be from configure.host not from the
> top-level, right?

That's right: for GCN, nvptx configurations inject '-fno-exceptions' etc.
via 'libstdc++-v3/configure.host'.

> But if we prepend, then when users put bad options in
> --enable-cxx-flags we silently override that with good target-specific
> ones from configure.host

That was my intention indeed -- but not a strong one.  ;-) (I've myself
never used '--enable-cxx-flags=[...]'.)

> Maybe if users explicitly give bad options, they should get an error,
> or a bad result?

That's fine for me.  So if you think that makes more sense, then I'll be
happy to swap it around.  I agree that it would be more standard to allow
user-specified flags to override the default ones.


Grüße
 Thomas


> Or maybe I just don't understand the context properly :-)
>
>
>>
>> >
>> >         libstdc++-v3/
>> >         * acinclude.m4 (GLIBCXX_ENABLE_CXX_FLAGS): Prepend any additional
>> >         flags to 'EXTRA_CXX_FLAGS'.
>> >         * configure: Regenerate.
>> >         * configure.host: Document 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'.
>> > ---
>> >  libstdc++-v3/acinclude.m4   | 3 ++-
>> >  libstdc++-v3/configure      | 3 ++-
>> >  libstdc++-v3/configure.host | 4 ++++
>> >  3 files changed, 8 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
>> > index b3423d7957a..3287dab3b89 100644
>> > --- a/libstdc++-v3/acinclude.m4
>> > +++ b/libstdc++-v3/acinclude.m4
>> > @@ -3269,7 +3269,8 @@ AC_DEFUN([GLIBCXX_ENABLE_CXX_FLAGS], [dnl
>> >      done
>> >    fi
>> >
>> > -  EXTRA_CXX_FLAGS="$enable_cxx_flags"
>> > +  # Prepend the additional flags.
>> > +  EXTRA_CXX_FLAGS="$enable_cxx_flags $EXTRA_CXX_FLAGS"
>> >    AC_MSG_RESULT($EXTRA_CXX_FLAGS)
>> >    AC_SUBST(EXTRA_CXX_FLAGS)
>> >  ])
>> > diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
>> > index e115ee55739..ba908577a66 100755
>> > --- a/libstdc++-v3/configure
>> > +++ b/libstdc++-v3/configure
>> > @@ -19452,7 +19452,8 @@ fi
>> >      done
>> >    fi
>> >
>> > -  EXTRA_CXX_FLAGS="$enable_cxx_flags"
>> > +  # Prepend the additional flags.
>> > +  EXTRA_CXX_FLAGS="$enable_cxx_flags $EXTRA_CXX_FLAGS"
>> >    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $EXTRA_CXX_FLAGS" >&5
>> >  $as_echo "$EXTRA_CXX_FLAGS" >&6; }
>> >
>> > diff --git a/libstdc++-v3/configure.host b/libstdc++-v3/configure.host
>> > index 45f55b250ce..1e84c78af30 100644
>> > --- a/libstdc++-v3/configure.host
>> > +++ b/libstdc++-v3/configure.host
>> > @@ -61,6 +61,10 @@
>> >  #
>> >  # It possibly modifies the following variables:
>> >  #
>> > +#   EXTRA_CFLAGS           extra flags to pass when compiling C code
>> > +#
>> > +#   EXTRA_CXX_FLAGS        extra flags to pass when compiling C++ code
>> > +#
>> >  #   OPT_LDFLAGS            extra flags to pass when linking the library, of
>> >  #                          the form '-Wl,blah'
>> >  #                          (defaults to empty in acinclude.m4)
>> > --
>> > 2.34.1
>> >